


Crystal Fun

by SinningfortheWinning



Category: Gravity Falls
Genre: Cock Worship, Come Inflation, Dirty Talk, F/M, Large Cock, Love Confessions, Pregnancy, Sibling Incest, Size Difference, Size Kink
Language: English
Status: Completed
Published: 2020-05-17
Updated: 2020-05-17
Packaged: 2021-03-03 04:15:24
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 1,643
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/24228688
Author URL: https://archiveofourown.org/users/SinningfortheWinning/pseuds/SinningfortheWinning
Summary: Dipper finds another size crystal in the woods and ends up using it to have some fun with Mabel.
Relationships: Dipper Pines/Mabel Pines
Comments: 2
Kudos: 77





	Crystal Fun

**Author's Note:**

> A request for Mal

The humid air in the woods made Dipper wish he could hop in the shower and give up on his hike. He wipes sweat from hiss forehead just as he passes into an area with crystals he remembers from his childhood.

Sizing crystals. Six years ago he had one strapped to a flashlight but that thing had been lost to time. So, he plucks another one from the ground, figuring he'll get some use of it.

With his new prize in hand, Dipper makes his way back home and takes a shower to wash the woods off of him. When he gets out he puts on shorts and lays in bed shirtless with the new crystal taped onto his flashlight.

He stares at it, wondering how he could entertain himself.

His answer comes through the door wearing a tight tank top and shorts that hugged her ass so well that Dipper wishes he was close enough to slap it.

"Hey, Dip," Mabel smiles. "I don't remember Gravity Falls being this hot." She fans her face with her hand, turned away from Dipper to look in her dresser for her swimming suit.

Dipper wonders if her shorts could handle it if her prefect ass grows a little. He points the flashlight at her, and what a great idea it was. Mabel let out a sound of surprise and looked back to watch as the light gave her shorts a test of strength.

The denim stretched as far as it could and barely held in her now pornstar sized rear.

"Dipper, you rascal!" she giggled. "I thought the view with my shorts would be enough."

Dipper shrugs and sits up. "It was, but I figured this would be better." He waved the flashlight in front of his face. "Found another one of these."

His twin tilts her head. "Oh? Let me see." He hands the light to her with a smirk. She smiles right back then points it at his entire body, only keeping it on for a moment. "Feel like a big man now?"

"Yeah, definitely bigger," he laughs. Dipper got out of bed and snatched the light back, flipping the crystal and using it to shrink Mabel an inch or two. "Now I'm the alpha twin," he says, resting his arm on her head like an arm rest.

"Psh, my butt stayed big." She wiggled her hips and moves out from under Dipper's arm to steal the light back. "I guess we can get specific with this, huh? Let's see..." She flipped the crystal, shining it on Dipper's crotch.

In his shorts, his flaccid dick sits at least an inch bigger. "Oh, you want to see specific?" The light is stolen back and Dipper shines it on his sister's breasts. He doesn't let up until they're bursting from her tank top. Then he flips the crystal and shrinks her further, her ass and tits both looking as if they were put into a little sex doll.

"Wow, I forgot how little it can make things," Mabel giggles. "Now what could you possibly want with me like this? I look like a big Barbie!"

"A Barbie that I'd like to play with." Dipper walked to their door and locked it. "Think we could...experiment like this?"

Mabel hums. "Make that cock of yours bigger and then we'll talk."

Dipper rolls his eyes but does as she asks, his cock now hard and a good sixteen inches and as thick as Mabel's arm. "There, just like you wanted." He lays on his bed undoing his shorts to free his new monster cock. Mabel eagerly lays between his legs.

His twin grins from ear to ear and quickly strips. She was about the size of a large doll and when she kissed the tip of his cock his precum alone covered her entire mouth. "Fuck, I want this stuffed into my cunt so bad."

Mabel kisses the head of his cock a few more times, inhaling through her nose as she went. "Your cock is so perfect, I bet it'll feel so good in me."

They hadn't ever actually fucked before. Mabel had "accidentally" given him a hand job once while looking through his pocket for the car keys and the two of them had whispered dirty things to each other in the night while masturbating. And they had showered together with heated looks. But it was clear that they were both tired of not getting the full show from each other.

"I'll gladly shove it into you right now," Dipper panted. "Want it in your throat or ass? I don't know if I should cum in your pussy."

"This cock can fill any of my holes." Mabel has her cheek pressed against the head of his dick, stroking it with both hands. "Your balls are so big now, I bet if you came in one hole the others would get some anyway."

Dipper shrugged. "Pussy it is." He helped his twin straddle him, lining up his massive cock with her pussy. "This is going to be so fucking tight. You sure it'll fit?"

Mabel eagerly spreads her legs and reaches down to rub her clit to get even wetter. "I don't care if it doesn't fit, I want this cock inside of me, please, Dipper I need it."

Nodding, Dipper uses his hand to push Mabel down onto his cock, her hips wonderful for helping him get the job done. Her body protests a little and he can see his cock bulging through her stomach.

"Deeper, Dipper please, fuck me deeper," his sister whined. He only had about six inches inside of her but if she wanted it deeper he'd get it deeper.

"I'm going to get it in there, don't worry." He applied more pressure to her hips, watching as his massive cock sunk deeper and deeper into his twin. When her big ass finally sat on his hips Dipper didn't even have to thrust because Mabel started to ride him.

All sixteen inches of his cock were massaged by her straining pussy, Mabel's small stomach protruding outward from it. Her large breasts were like two big water balloons bouncing on her chest and her big ass jiggled each time she moved her hips. Dipper can hardly decide which one he wants to grope so he uses one hand to grab a boob and the other to get a handful of ass.

"Your pussy feels amazing," he groans. "We're not going to get any sleep during this trip. I want to stay up here and fuck you all night."

"Your cock feels even better." Mabel's voice comes out higher pitched than normal, his twin clearly enjoying herself. "Oh God, I want you inside of me constantly. Dipper I've wanted to fuck you like this for so long, I'll stay in your bed forever if you just fuck my brains out with this wonderful cock!"

Dipper can hardly believe Mabel can speak so clearly with so much dick in her, but the promise of fucking more often makes his cock twitch hard enough that he can see it through her stomach.

"Yeah, ride me and tell me how much you want your brother's cock, c'mon baby, keep it up." Dipper squeezes her tit and ass, thrusting up to meet her hips when she brings them down. He lets go of her ass for a moment to grab the light again, angling it at his balls. They grow so large that he can barely close his legs.

"There, now I'll definitely fill you up." Dipper points the light at Mabel's ass again then her chest, then shrinks her as she rides him. Her body looks like a customized sex doll and the more he shrank her the tighter her pussy was on his cock.

"Like that? Like having your entire body used like a cock sleeve for your brother?"

"Yes, yes, I can feel it all over! Use me like a fleshlight, let your wonderful cock fill my cunt until I burst!" Mabel is working her entire body now, her eyes rolled back into her head. Dipper's cock looks like it could break her in half at any moment but that doesn't stop him from grabbing her hips and fucking into her.

Mabel's mouth opens and her tongue rolls out, a smile at the corners of her lips. "Harder, Dipper harder! Use your sister like the cum dumpster I am!"

"I'm going to keep coming in you until these big balls are empty," he grunted out. The release of his orgasms is close and when it finally hits him like a wall Dipper slams all sixteen inches into his twin and watches in interest as her stomach begins to fill.

At first, it doesn't seem like much happened. But then her belly began to bulge outwards and her entire body jerked each time a jet of cum came from his balls.

"Oh fuck, it's so much, I'm going to get pregnant! Dipper, keep filling my pussy until I'm swollen with your baby! Please, I love you, I want it so bad!" Mabel was coming as he was, unable to move from how heavy her stomach was becoming.

"I love you too," Dipper panted. "There's so much, take it all for me, okay?" He gripped the blanket and tilted his head back and enjoying his prolonged orgasm.

By the time he was done, Mabel's stomach looked as if she were pregnant with twins. Which after this she probably was. "That was the best fuck of my life," she laughed, voice strained. "Don't know how I'll manage sleeping in my own bed ever again."

"Me either," Dipper laughed. "Looks like we're in this mess for the long haul." He sat up and pulled her close, kissing her briefly. "I'm glad we did this."

His twin smiled. "Me too."

**Author's Note:**

> If you have a request leave it below. Can't promise I'll do it but I'll do the good ones and ones I have time to do.


End file.
